Amplification underlies the functioning of many biological and engineering systems. Simple electrical, optical, and mechanical amplifiers are usually reciprocal, that is the backward coupling of the output to the input equals the forward coupling of the input to the output. Unidirectional amplifiers are special non-reciprocal devices in which the output does not couple back to the input even though the forward coupling persists. Examples of unidirectional amplifiers include operational amplifiers in electrical engineering and magneto-optical devices in microwave technology. Semiconductor components are used in electrical circuits and Faraday rotation is used in optical systems to violate reciprocity and create unidirectional amplifiers. Unidirectional coupling has not been implemented, however, in a mechanical system.
Mechanical amplification can enhance the detection of a weak signal by raising its amplitude above the noise level. Biology employs this strategy in hearing: mechanosensitive hair cells in the vertebrate inner ear actively amplify weak sounds and thereby greatly lower the threshold of hearing. In contrast, microphones—the ear's technological analogue—are passive devices that do not employ mechanical amplification but rely on subsequent electronic signal processing. One difficulty in implementing mechanical amplification in microphones is the reciprocity described above, which leads to undesired feedback and hence highlights the need for a mechanism to implement unidirectional mechanical amplification.